WO2000075419A1

WO2000075419A1 - Distribution device for a device for dewatering pulp

Info

Publication number: WO2000075419A1
Application number: PCT/SE2000/001108
Authority: WO
Inventors: Stefan Hansson
Original assignee: Kvaerner Pulping Ab
Priority date: 1999-06-04
Filing date: 2000-05-30
Publication date: 2000-12-14
Also published as: SE512877C2; SE9902061L; SE512877E; SE9902061D0; US6668600B1

Abstract

Device for washing and dewatering a fibre pulp suspension, which device comprises two hollow, circular-cylindrical screen members (1), which screen members contain evacuation chambers, internally in the screen members, for conducting away liquid. The screen members rotate towards each other for the purpose of forming a nip (2), with at least one of the said screen members (1) being arranged in a trough (7, 8) which partially encloses the casing (3) of the screen member and which converges towards the casing of the screen member in the direction of rotation of the screen member. At least one pulp inlet box (4) is arranged at the highest point (1) of the screen members, which are provided with troughs, for the purpose of introducing pulp between the casing (3) of the screen member and its trough (7, 8) for the purpose of forming a fibre pulp web. The pulp inlet box is provided with a pressure equalization section in which a certain equalization of irregularities in pulp qualities takes place before the pulp is fed into the actual washing or dewatering gap. The invention provides improved operability, especially at high pulp concentrations, and reduces the risk of plug formation.

Description

DISTRIBUTION DEVICE FOR A DEVICE FOR DEWATERING PULP

The present invention relates to a distribution device m accordance with the preamble to Claim 1.

STATE OF THE ART

When producing paper pulp from cellulose- containing fibre material, there is a need to wash and dewater the paper pulp at one or more points m the process.

A previously known and frequently employed device, termed a wash press, for washing and dewatering paper pulp is shown in SE-C-380 300, SE-C-501 710, US 5,488,900 and SE-C-504 011. The devices which are shown m these publications comprise two cylindrical, rotatable screen members which are arranged m a trough which is m the mam convergent but which is partially divergent at the intake for washing liquid. Other examples of known devices are shown m US 4,543,161 and US 5,667,642, with the latter constituting a device m which the screen members rotate in the opposite direction to the conventional direction, i.e., when seen from a short side, the right screen member rotates anticlockwise and the left rotates clockwise. A problem associated with devices of the prior art, which possess two cylindrical screen members which can be rotated towards each other, is that of obtaining an efficient initial distribution of pulp over the whole of the length of the dewatering device. An example of how this problem has been solved m shown m SE-C-500 546, m which a crescent is to distribute the pulp over a line by means of conducting the pulp from a punctate inlet and over the edge of the crescent. The lowest height of the crescent is obtained furthest away from the inlet and consequently a longer supply distance is compensated by a lower bridging height. In these types of solution, practical limits have been set at pulp concentrations of the order of 8%.

Other solutions have instead employed a distribution screw, similar to US 4,559,104, which distribution screw provides a substantially improved initial distribution over the whole of the width of the wash press. A good initial distribution of the pulp ever the whole of the width of the wash press is a very important parameter for succeeding m obtaining a wash press which possesses nigh operability, i.e. which αces not plug up, with the press subsequently having to be stopped m order to remove plugs, and a high dewatering ability. Ways of dealing with the problems involved m assisting uniform distribution of the pulp over the whole of the width of the wash press have exclusively concentrated on solutions corresponding to SE-C-500 546 or US 4,559,104, and it has only been possible to discern any improvements in the transitions between pulp inlet boxes and trough sweeps m the form of shorter radii between the pulp inlet boxes and the trough sweeps, corresponding, for example, to US 5, 667, 642.

In some wash presses, use has instead been made of a very long, gradual decrease m the gap from the inlet box, see, for example, SE 380 300. While tms provides an equalizing effect, this very lengthy dewatering and equalizing zone at the same time results m a reduced total displacement ability. That is to say that the washing liquid which is to penetrate through the pulp web is given a much shorter stretch to act cn, and the pulp web which is to be washed becomes thicker. An inferior displacement ability results m higπer residual contents of chemicals m the pulp, sometnmg which is a negative feature, chiefly from the environmental point of view and from the point of v ew of recovery.

BRIEF DESCRIPTION OF THE INVENTION

One aim of the present invention is to provide a wash press whose operability is improved and which can be fed with high concentrations of pulp without there being any risk of it plugging up. When use has been made of a pulp inlet box m accordance with the invention, with the inlet box being employed for a wash press having double circular-cylindrical screen members and pulp inlet boxes at the highest point of the screen member, good operability has been obtained with pulp concentrations of the order of 12% m the inlet.

An additional aim is for the pulp web to be relatively rapidly reduced m thickness, thereby providing the possibility of improved washing, i.e. improved displacement ability. It is important to be able to use new washing liquid to rapidly displace the chemicals which are present m the pulp, with these chemicals being returned, where appropriate, to the preceding stage m the pulp preparation process. A device m accordance with the characterizing part of Patent Claim 1 is consequently presented m accordance with the invention.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described below with reference to the figures, of which:

Fig. 1 shows a distribution device according to the invention which is implemented in a wash press and which is seen m cross section.

Fig. 2 shows the distribution device according to the invention on a larger scale.

DETAILED DESCRIPTION OF THE FIGURES The preferred embodiment of the device according to the invention is shown m Fig. 1 and comprises two hollow, circular-cylindrical screen members 1 which contain a number of evacuation chambers, under the casing surface of the screen member, for conducting away evacuated liquid. The two screen members form a press nip 2 between each other and are arranged to rotate towards each other, with, when seen from the short side, the right screen member rotating clockwise and the left screen member rotating counterclockwise. Since the device is m the main symmetrical m a plane of symmetry which is formed by a tangent to the screen members 1 m the nip 2, only one of the symmetrical parts will m principle be described from now on.

The diameter of the screen member 1 is expediently 1.0-2.5 metres. Its casing 3 is furthermore perforated in order to enable liquid to be evacuated from a fibre pulp web which abuts the casing surface and then on into individual evacuation chambers 22, which conduct evacuated liquid away in an axial direction m the screen member. The evacuation chambers 22 communicate with each other by channels, which run m a peripneral direction, being formed between supports (not shown) which are arranged directly under the screen plating resting against the axially directed evacuation chambers .

In the preferred embodiment which is shown m Fig. 1, a pulp inlet box 4 is arranged at each screen member 1. Each pulp inlet box 4 is arranged at 0° m relation to the screen member, with 0° being the highest/topmost point of the screen member and the number of degrees increasing positively m the direction of rotation of the screen member. The inlet box distributes the incoming paper pulp, whose concentration is normally about 1-12%, expediently 3-10%, uniformly over the length of the screen member. A longitudinal gasket 5, which bears against the casing 3 of the screen member and which prevents liquid running from the incoming pulp suspension, against the direction of rotation, and down into outgoing pulp which has already been dewatered, is arranged at the rear edge of the inlet box. A spray 6 is arranged to wash away fibres which may possibly collect on the gasket 5 and to clean the holes or slits m the casing 3 of the screen member. The gasket 5 is expediently constructed such that some fibres which may remain on the screen member are allowed to pass beneath the gasket but nevertheless such that it provides good sealing against the pulp m the inlet box 4.

In the preferred embodiment shown m Fig. 1, a trough is furthermore arranged, which trough consists, for each screen member 1, of at least two trough sweeps 7, 8. The top trough sweep 7 encloses the casing of the screen member from the pulp inlet box 4, where, m the operating position, the top trough sweep is m the main arranged so that it seals against the pulp inlet box, or as shown, against a short piece of the top trough sweep 10, which is built as one piece with the pulp inlet box. In the preferred embodiment, the top trough sweep has a lower end point 11 at about 115° (m the operating position) and can be pivoted around an axle 9, which axle is parallel to the casing surface 3 of the screen member and is arranged m the vicinity of the said end point 11. When the top trough sweep is to be pivoted, for access for, for example, cleaning the screen member, a hydraulic cylinder 12 folds the trough sweep outwards by way of a lever arm between the axle 9 and the attachment point 13 of the hydraulic cylinder in the top trough sweep. The top trough sweep is reinforced with external flanges 14, whose number is expediently matched to the width of the trougn, which extend along the circle segment of the top trough sweep 7 and which are provided with additional reinforcing, transverse struts 15.

At the end point 11 of the top trough sweep, it bears, m the operating position, m a sealing manner against the bottom trough sweep 8. In this context, the bottom trough sweep 8 extends from the end point of the top trough sweep, along the casing surface 3 and through to the nip 2. It will be understood that the bottom trough sweep 8 cannot reach right into the nip, m association with which it expediently finishes m a position at which the distance between the two screen members 1 corresponds to about twice the gap width m the end point of the bottom trough sweep 8. A number of washing zones 18, which contain inlets (not shown) for washing liquid, are arranged m the bottom trough sweep 8. In the embodiment snown, three longitudinal washing zones 18 have been arranged at about 140°, 170° and 200°, respectively. While the number of washing zones is matched to each respective application, 2-3 washing zones are normally employed between approximately 115° to approximately 200°. The bottom trough sweep 8 can be lowered for access and cleaning the casing 3 of the screen member. The bottom trough sweep 8 is lowered using a suitable device, preferably a hydraulic positioning device.

A gap 19 is present between the casing surface 3 of the screen member and the trough 7, 8, which gap is arranged to converge from the pulp inlet box 4 to the nip 2, with, however, it being possible for divergent areas to occur m places, for example at washing zones 18, where washing liquid is to be introduced into the fibre pulp web which is present m the gap. The width of the gap between the walls of the trough and the casing 3 can be regulated so as to achieve optimal dewatering and is adjusted depending on the concentration of the incoming fibre pulp and the desired degree of dewatering. At the inlet, the gap width is typically of the order of size of 50-150 millimetres, while the gap width at the outlet is typically of the order of size of 10-40 millimetres. Above the nip 2 is arranged a stripper and conveyor worm 20 which strips off the washed and dewatered fibre pulp web and conveys t away for further treatment m the process for manufacturing paper pulp. Alternatively, or as a complementary addition, a scraper can also be used for detaching the fibre pulp web from the casing 3.

During operation, a paper pulp whose concentration is about 1-12% is conducted into the gap 19 via the pulp inlet box 4. The invention is now described m more detail with reference to Figure 2, which shows the essential features of the invention. The pulp inlet box 4 distributes the pulp over the whole width of the wash press, in a manner known per se using a distribution screw corresponding to that shown in US 4,559,104, after which it is fed down towards the circular-cylindrical casing surface of the screen member, via the gap 30, to a first section. The gap 30 is open radially outwards towards the pulp inlet box such that pulp can be conducted to the gap 30 continuously. Directly after this initial supply of the pulp to section 1, a second section of the distribution device begins, which second section is closed at a first distance A calculated from the casing surface of the screen member and m a radial direction out from the centre of rotation of the circular- cylindrical screen member. The second section is arranged after the first section, seen m the direction of rotation of the screen member. In this second section, the pulp is equalized over the width of the wash press while continuously being drawn m towards the trough gap. The distance between the casing of the screen member and the radially external delimitation surface corresponds to the distance A m Figure 2, and, at least at the beginning of the equalization zone m the second section, this distance is at least 50% greater than that of the subsequent trough sweep gap B. The distance A is expediently ad usted to the prevailing pulp concentrations, with A as a rule decreasing as the concentration of the pulp which is conducted to the gap 30 increases. However, it is expedient for the first distance A to be within the interval 150-800% of the second distance B and preferably 500%. In order to ensure that a good equalization effect is obtained, the equalization section should also extend peripherally over an angular distance α which exceeds 5°. The angular distance α expediently lies within the interval 5-30°, preferably 20°. Too long an equalization zone results m the available washing and displacement zones decreasing, and the size of the equalization zone is consequently a compromise between a good initial distribution and washing and displacement results.

The screen members 1 are arranged to rotate at a speed of 5-20 rpm using a suitable drive device. The paper pulp then accompanies the rotation of the screen members, passing into the gap 19 between the perforated casing surface 3 and the walls of the trough 7, 8, m association with which it forms a fibre pulp web which is dewatered due to the gap converging m the direction towards the nip. The liquid which is pressed out of the fibre pulp web is conducted away (not shown) from the device. Washing liquid is introduced into the fibre pulp web at the washing zones 18, where the gap can be somewhat divergent, with washing of the web then taking place. Finally, the pressure m the nip 2 dewaters the fibre pulp web to a concentration which is about 5-20 times higher than the concentration of the incoming paper pulp, for example 1-12% when supplied and 25-40% after the nip. The stripper and conveyor worm 20 strips the fibre pulp web off the casing 3 and conveys it away from the device.

The device according to the invention is not limited by the embodiments which are described above but can be varied within the scope of the subsequent patent claims. For example, the radially external delimitation wall of the equalization zone can consist of smooth sheet metal, with the distance A being continuously reduced to the distance B over the equalization zone. The equalization zone can also, as shown m Figures 1 and 2, be provided with a radially external delimitation wall which, at one bound, halfway into the equalization zone, is angled off more sharply, resulting m a more rapid reduction in the gap being obtained during the later phase of the equalization zone.

Claims

PATENT CLAIMS

1. Device for washing and dewatering a fibre pulp suspension, which device comprises at least one but preferably two circular-cylindrical screen members (1) which are arranged to rotate towards another circular- cylindrical member, preferably a circular-cylindrical screen member, for the purpose of forming a nip (2), with at least one of the said screen members being hollow, allowing evacuation of liquid radially inwards into the screen member, with at least the hollow screen member (1) being arranged m a trough (7, 8) which partially encloses the casing (3) of the screen member and which, m the direction of rotation of the screen member, converges towards the casing of the screen member, and with at least one pulp inlet box (4) being arranged at the screen member (1), which is provided with a trough, for introducing pulp between the casing

(3) of the screen member and its trough (7, 8) for the purpose of forming a fibre pulp web, c h a r a c t e r i z e d

- m that the pulp inlet box contains a first section and m that this first section (30) is open m a radial direction outwards from the centre of rotation of the circular-cylindrical screen member for the purpose of receiving pulp which is conducted to the pulp inlet box,

- in that the pulp inlet box contains a second section and in that this second section is closed at a first distance (A) from the casing surface of the screen member m a radial direction outwards from the centre of rotation of the circular-cylindrical screen member, and m that this second section is arranged after the first section, seen in the direction of rotation of the circular-cylindrical screen member,

- m that the second section merges into a trough sweep which is arranged initially at a second distance (B) from the casing surface of the screen member after the second section, seen m the direction of rotation of the circular-cylindrical screen member,

- and m that the first distance exceeds the second distance by at least 50% and m that the second section extends over at least 5° over the periphery of the screen member.

2. Device according to Claim 1, c h a r a c t e r i z e d i n that the second section extends over the periphery of the screen member within the interval 5-30°, preferably 20°.

3. Device according to Claim 1 or 2, c h a r a c t e r i z e d i n that the first distance (A) lies within the interval 150-800% of the second distance, preferably 300%.

4. Device according to any one of the preceding claims, c h a r a c t e r i z e d i n that the second section merges into a trough sweep which is arranged initially at a second distance (B) from the casing surface of the screen member after the second section, seen in the direction of rotation of the circular-cylindrical screen member, and m that this transition between the second section and the trough sweep preferably takes place continuously.

5. Device according to any one of the preceding claims, c h a r a c t e r i z e d i n that the pulp inlet box (4) is arranged to connect to the highest point of the circular-cylindrical screen member.